/Engine/Plugins/Protocols/MQTT/Source/MQTTCore/Public/MQTTClientObject.h
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintCallable, Category="MQTT") void SetOnMessageHandler ( const [FOnMessageDelegate](API\Plugins\MQTTCore\UMQTTSubscriptionObject\FOnMessageDelegate) & OnMessageCallback )